It is well known to draw and iron a sheet metal blank to make a thin walled can body for packaging carbonated beverages, non-carbonated beverages, food or other substances within the can body. In a conventional can body, a bottom profile of the can body includes an outwardly protruding annular ridge near the periphery of the can body, and a slope inwardly from the annular ridge that forms an inwardly projecting dome portion of the can body, called a domed can end.
In a two-piece can, the dome would be formed directly into the bottom of the can body with the dome integrally connected to the drawn and/or ironed sidewalls of the can body. The can is completed by seaming a can end or affixing a closure on the top of the can body for sealing the contents of the can body.
In a three-piece can, the dome would be formed in a can end. The domed can end is then seamed onto the bottom of the can body. Like the two-piece can, the three-piece can is completed by seaming a can end or affixing a closure on the top of the can body for sealing the contents of the can body.
Domed can ends significantly increase the strength of the bottom of a can body. This allows manufacturers to reduce the amount of metal used in the domed can end without sacrificing strength. Additional advantages of doming includes an increased ability to stack cans on top of each other when a domed can end is present on the can body.
Several prior U.S. patents disclose apparatus and methods wherein domed can bodies are formed. These patents generally disclose apparatuses with a curved, convex punch core and a concave die core, such that a domed can body is formed from material conveyed between the punch core and the die core. Typically, the punch core extends downward into the die core, forming the domed can body. These patents, however, generally include a single action of pushing the punch core into the die core. Representative patents include U.S. Pat. No. 6,070,447 to Bone et al., U.S. Pat. No. 5,154,075 to Hahn et al., and U.S. Pat. No. 4,723,433 to Grims.
There continues to be a need in the art for forming domed can bodies and can ends that are substantially free from wrinkles that are formed during drawing and doming. Wrinkles can cause problems in the finishes in the dome of the can body and/or can end, for example, by causing cracks in the epoxy placed on the can body and/or can end. Several patents disclose methods and apparatus to overcome these wrinkles. For example, U.S. Pat. No. 4,685,322 to Cloves discloses a method for reducing wrinkling by forming an upwardly projecting annular bead in a bottom wall of a cup that is subsequently redrawn into a can body. U.S. Pat. No. 4,372,143 to Elert et al. discloses a method wherein a blank is preformed into a cup and thereafter is held in place by pressure sleeves on a die core while the inwardly projecting domed surface is formed by the die core. U.S. Pat. No. 5,394,727 to Diekhoff et al. discloses a method that includes the steps of forming a blanked sheet into a cup, forming a recessed boss into the base of the cup, redrawing the cup into a redrawn cup, and ironing and reforming the redrawn cup into a can body. Wrinkling is reduced by controlling the metal flow during the redraw until a lower body radius and a redraw radius are approximately tangent.
These wrinkling-decreasing techniques utilize multiple steps to shape and redraw the material prior to forming the domed shape in the material and form the dome by utilizing a single convex domed-shaped forming tool entering into a corresponding concave tool. As such, room remains in the art for an alternative dome forming method and apparatus that does not unduly strain the material being formed into a domed can end or can body.